PURPOSE: We studied the clinical features and the etiology of hepatitis B virus surface antigen (HBsAg)-negative and antibody to hepatitis C virus (anti-HCV) negative patients with hepatocellular carcinoma. METHODS: A total of 550 patients, hospitalized with an initial diagnosis of HCC were retrospectively studied. Eighty-one of these patients were HBsAg-positive (HB group), 404 patients were anti-HCV positive (HC group). The other 65 patients were negative for both HBsAg and for anti-HCV (NBNC group). We purified HBV-X gene from HCC or non-tumorous liver tissue of 23 NBNC patients using PCR. RESULTS: Clinical features of NBNC as compared with HB and HC patients were as follows, respectively: non-cirrhosis rate (%): 57,37,15 (P = 0.02 for HB, P < 0.00001 for HC), the proportion of patients with normal ALT concentrations (%): 59,28,10 (P = 0.0002 for HB, P < 0.00001 for HC). Forty of 59 NBNC patients (68%) had anti-HBs and/or anti-HBc (healthy controls: 29%, P < 0.00001) and two of 56 had serum HBV DNA. Twelve of 23 NBNC patients had HBV-X gene in HCC and/or non-cancerous liver tissues (52%). None of 52 had serum HCV RNA. CONCLUSIONS: The NBNC patients with HCC had a higher frequency of non-cirrhotic liver without liver injury. The presence of the HBV-X gene in HCC suggests a possible role of past HBV infection in the development of HCC. About half of NBNC HCC is associated with seronegativity for HBsAg and positivity for the HBV-X gene in liver tissue.